1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and a method for fabricating the same, and more particularly, to a polycrystalline silicon thin film transistor (poly-Si TFT) and a method of fabricating the same that decreases leakage current and improves electrical characteristics of the TFT.
2. Discussion of the Related Art
In general, a poly-Si TFT has high carrier mobility, decreased photo current, and relatively low-level shift voltage in comparison with an amorphous silicon thin film transistor (a-Si TFT). Accordingly, a poly-Si TFT is commonly employed as a switching element in the liquid crystal display panel to achieve high resolution or in a projection panel to achieve high light intensity. Further, a poly-Si TFT can be fabricated as both an n-channel TFT and a p-channel TFT to form a CMOS circuit.
In addition, a poly-Si TFT can be fabricated by utilizing current semiconductor fabrication processes, because the method for fabricating a poly-Si TFT is similar to the CMOS process of silicon wafer. In particular, an intrinsic amorphous silicon layer is formed on an insulating substrate by using a Plasma Chemical Vapor Deposition (PCVD) method or a Low Pressure Chemical Vapor Deposition (LPCVD) method. After the amorphous silicon layer has a thickness of about 500 angstroms (Å), it is re-crystallized into a polycrystalline silicon layer using a crystallization method.
The crystallization method is generally classified into one of an Excimer Laser Crystallization (ELC) method, a Solid Phase Crystallization (SPC) method, a Metal Induced Crystallization (MIC) method, and a Metal Induced Lateral Crystallization (MILC). In the ELC method, an insulating substrate having an amorphous silicon layer formed thereon is heated to a temperature of about 250° C. Then, an excimer laser beam is applied to the amorphous silicon layer to form a polycrystalline silicon layer. In the SPC method, the amorphous silicon layer is heat-treated at a high temperature for a long time to crystallize into a polycrystalline silicon layer.
In the MIC method, a metal layer is deposited on the amorphous silicon layer and the deposited metal is used for crystallization. Here, a large-sized glass substrate can be used as an insulating substrate. In the MILC method, a metal is first formed on the amorphous silicon layer, and then the amorphous silicon layer is crystallized. Also, in the MILC method, an oxide pattern is formed on a predetermined active portion of the amorphous silicon layer, and the amorphous silicon layer is converted into polycrystalline silicon by lateral grain growth.
Since the ELC method can be performed at a relatively low temperature on inexpensive glass substrates, the ELC method has been widely used to covert amorphous silicon into polycrystalline silicon by applying laser energy to the deposited amorphous silicon,. Further, when the ELC method is used to form TFTs that are adopted as switching elements in an array substrate of a liquid crystal display, the fabricated TFTs become n-channel TFTs and manipulate liquid crystals by applying voltages to the liquid crystals.
To have a high display quality in a liquid crystal display, the TFT is required to have a sufficiently low OFF current (i.e., a current flowing when the TFT is turned OFF). However, the poly-Si TFT has high ON and OFF currents in comparison to an a-Si TFT. Since the carrier mobility of the polycrystalline silicon is large. Thus, leakage current increases in an interface between doped source and drain regions and un-doped channel region.
To solve the problems above, the polycrystalline silicon layer of the poly-Si TFT has an offset area or a lightly doped region (LDR) in the source and drain regions. The offset area is an un-doped region of the source and drain regions and the LDR is a region that impurities of low concentration are lightly doped. Additionally, the gate electrode of the poly-Si TFT has a multiple structure, e.g., a dual structure, to decrease the leakage current.
FIGS. 1A to 1E are cross-sectional views of a polycrystalline silicon thin film transistor according to the related art. In FIG. 1A, a buffer layer 12 is first formed on a substrate 10. The buffer layer 12 is a silicon insulating material, such as silicon nitride (SiNx) or silicon oxide (SiO2), and the buffer layer 12 functions to prevent diffusion of alkali material from the substrate 10 when heat is applied to the substrate 10. Also, amorphous silicon, such as a-Si:H, is deposited on the buffer layer 12 to form an amorphous silicon layer 14. Then, the substrate 10 having the amorphous silicon layer 14 is heated to a temperature of 400 to 500° C. to eliminate hydrogen gas (H2) included in the amorphous silicon layer 14, wherein the heating process is commonly known as a dehydrogenation process. In FIG. 1B, after the dehydrogenation process, laser beams are applied to the amorphous silicon layer 14 shown in FIG. 1A, thereby converting the amorphous silicon layer 14 to a polycrystalline silicon layer 16.
In FIG. 1C, the polycrystalline silicon layer 16 (in FIG. 1B) is then patterned into an island shape to form an active layer 18. In addition, a gate insulation layer 20 is deposited on the buffer layer 12 to cover the active layer 18. The gate insulation layer 20 is an inorganic material, such as silicon nitride (SiNx) or silicon oxide (SiO2). Further, a conductive metallic material is deposited on the gate insulation layer 20, and then patterned to form a gate electrode 22 over the active layer 18. The deposited conductive metallic material may be aluminum (Al), chromium (Cr), molybdenum (Mo), or molybdenum tungsten (MoW). Moreover, the gate electrode 22 can have a double-layered structure of molybdenum/aluminum neodymium (Mo/AlNd). A portion of the active layer 18, which corresponds to the gate electrode 22, is a channel region when the thin film transistor is complete. Alternatively, the gate electrode 22 can be made of polycrystalline silicon. After forming the gate electrode 22, a low-density n-type dopant (hereinafter referred to as a n− ion) is applied to an entire surface of the substrate 10. However, the gate electrode 22 functions as a mask, such that the active layer 18 is doped by the n− ion except for the portion corresponding to the gate electrode 22.
In FIG. 1D, a photo resist pattern 23 is formed on the gate insulation layer 20 to cover the gate electrode 22. Therefore, the doped portion of the active layer 18 is considered as divided into a first area A1 and a second area A2, where the first area A1 is overlapped by the photo resist pattern 23 and the second area A2 is not overlapped by photo resist pattern 23. After forming the photo resist pattern 23, a high-density n-type dopant (hereinafter referred to as a n+ ion) is applied to the entire surface of the substrate 10. Then, the first area A1 of the active layer 18 becomes a lightly doped region (LDR), and the second area A2 of the active layer 18 becomes a highly doped region (HDR), thereby forming source and drain regions. Accordingly, the substrate 10 includes, on both sides of the gate electrode 22, the source and drain regions A2 where the high density n-type ions are doped and the LDRs A1 where the low density n-type ions are doped.
In FIG. 1E, the photo resist pattern 23 (in FIG. 1D) is removed, and a passivation layer 24 is formed on an entire surface of the gate insulation layer 20 to cover the gate electrode 22. The passivation layer 24 is an inorganic material, such as silicon nitride (SiNx) or silicon oxide (SiO2), or an organic material, such as benzocyclobutene (BCB) or an acrylic resin. Then, the passivation layer 24 and the gate insulation layer 20 are partially etched to form a source contact hole 26 and a drain contact hole 28 exposing the source and drain regions A2, respectively. Thereafter, the source and drain electrodes 30 and 32 are formed on the passivation layer 24, where the source and drain electrodes 30 and 32 contact the source and drain regions A2, respectively, through the source and drain contact holes 26 and 28. Accordingly, a poly-Si TFT is formed with the LDRs in the source and drain regions of the active layer and multiple gate electrodes (e.g., a dual gate electrode) to more decrease the leakage current. When the multiple gate electrodes are employed in the poly-Si TFT, the LDRs are enlarged and the electric field decreases in the TFT, thereby lowering the leakage current.
FIGS. 2A to 2D are cross-sectional views of another polycrystalline silicon thin film transistor having dual gate electrodes according to the related art. In FIG. 2A, a buffer layer 52 is formed on a substrate 50. The buffer layer 52 is a silicon insulating material, such as silicon nitride (SiNx) or silicon oxide (SiO2). Also, a polycrystalline silicon layer is formed on the buffer layer 52, and then patterned to form an island-shaped active layer 54 of polycrystalline silicon.
In FIG. 2B, a gate insulation layer 56 is formed on the buffer layer 52 to cover the active layer 54, and the gate insulation layer 56 is an inorganic material, such as silicon nitride (SiNx) or silicon oxide (SiO2). After forming the gate insulation layer 56, a dual-gate electrode 58 is formed on the gate insulation layer 56 over the active layer 54. The dual-gate electrode 58 includes a first gate electrode 58a and a second gate electrode 58b, wherein both the first and second gate electrodes 58a and 58b receive the same voltage. Thereafter, the n− ion is doped an entire surface of the substrate 50. Thus, the active layer 54 is doped by the n− ions except the portions overlapped by the first and second gate electrodes 58a and 58b where the dual-gate electrode 58 functions as a mask. Further, a portion of the active layer 54 between the first and second gate electrodes 58a and 58b becomes a first active area B1, and the outer parts of the active layer 54 which are doped by the n− ions become second active areas B2.
In FIG. 2C, photo resist patterns 60a and 60b are formed on the gate insulation layer 56 while covering the first and second gate electrodes 58a and 58b. The first photo resist pattern 60a covers and surrounds the first gate electrode 58a, and the second photo resist pattern 60b covers and surrounds the second photo resist pattern 58b. Since the first and second photo resist patterns 60a and 60b are not connected to each other, the first active area B1 is divided into third active areas B3 that the first and second photo resist patterns 60a and 60b overlap and a fifth active area B5 over which the photo resist pattern 60 do not exist. Accordingly, each second active area B2 is divided into the third active area B3 over which the photo resist pattern 60 exists and a fourth active area B4 over which the photo resist pattern 60 does not exist.
After forming the photo resist patterns 60a and 60b, n+ ions, such as phosphorous ions, are applied to the entire surface of the substrate 50. Therefore, the fourth and fifth active areas B4 and B5 become highly doped regions (HDRs), and the third active areas B3 overlapped by the photo resist patterns 60a and 60b become lightly doped regions (LDRs), thereby forming source and drain regions. After the n+ ion doping, the photo resist patterns 60a and 60b are sequentially removed. Accordingly, the active layer 54 includes the LDRs around the dual-gate electrode 60, and the HDRs around the LDRs.
In FIG. 2D, a passivation layer 62 is formed on the entire surface of the gate insulation layer 56 to cover the dual-gate electrode 58. Then, the passivation layer 62 and the gate insulation layer 56 are partially etched to form a source contact hole 64 and a drain contact hole 66. The source contact hole 64 and the drain contact hole 66 expose the highly doped source and drain regions B4, respectively. Thereafter, source and drain electrodes 68 and 70 are formed on the passivation layer 62. The source and drain electrodes 68 and 70 contact the source and drain regions B4, respectively, through the source and drain contact holes 64 and 66. Accordingly, a polycrystalline silicon thin film transistor is formed having the dual-gate electrode and the LDRs in the active layer.
However, when forming the photo resist pattern for the LDRs according to the related art, the photo resist pattern may be misaligned due to fabrication errors. As a result, the LDRs disposed on both sides of the gate electrode may have different sizes. If the LDRs are disposed asymmetrically in the active layer, the poly-Si TFT may have an unstable and swaying threshold voltage.